An ion guide may be utilized to transmit ions in various types of ion processing devices, one example being a mass spectrometer (MS). The theory, design and operation of various types of mass spectrometers are well-known to persons skilled in the art and thus need not be detailed in the present disclosure. A commonly employed ion guide is based on a multipole electrode structure, which may be a RF-only electrode structure in which the ions passing through the ion guide are subjected to a two-dimensional RF electric field that focuses the ions along an axial path through the electrode structure. A DC offset component may also be added to modify the axial energy or focusing conditions of the ion beam.
A curved ion guide is one in which the ion axis along which the ions pass is a curved path rather than a straight path. A curved ion guide is often desirable for implementation in ion processors such as mass spectrometers because the curved ion guide can improve the sensitivity and robustness of the mass spectrometer. A primary advantage of the curved ion guide in such a context is that it provides a line-of-sight separation of the neutral noise, large droplet noise, or photons from the ions, thereby preventing these components from reaching the more sensitive parts of the ion optics and ion detector. Moreover, the curved ion guide enables the folding or turning of ion paths and allows smaller footprints in the associated instruments.
As appreciated by persons skilled in the art, in a curved ion guide the ions are transmitted around a curved ion path through oscillations inside the radial trapping field provided by the RF voltage applied on the rods (i.e., electrodes) of the ion guide. In the absence of the RF field, the ions would move straight and eventually hit the ion guide rods. Therefore, in the curved ion guide the ions need to experience a certain minimum amount of RF restoring force during their flight before they move too close to the ion guide rods and become unstable. When the ion guide transmits one mass at a time, the best performance is obtained when the RF voltage is scanned as a function of mass to optimize transmission. However, it is often desirable to run ions at higher energy and/or transmit ions of multiple different masses (mass-to-charge, or m/z, ratio) simultaneously. In such cases, some of the ions cannot have optimal transmission conditions and they are lost, leading to less than optimal instrument sensitivity.
Accordingly, there has been a need for improved curved ion guides, including ion guides capable of transmitting ions at high levels of kinetic energy and simultaneously transmitting ions of multiple masses while maintaining optimized ion transmission conditions. This need is addressed in U.S. patent application Ser. No. 12/277,198, assigned to the assignee of the present disclosure. The foregoing patent application discloses the application of a deflecting DC electric field on the ion guide in the radial direction toward the center of the ion guide sector, to compensate for the ion kinetic energy and assist in deflecting ions around the curved geometry. The applied radial deflecting electric field may be a function of the ion axial kinetic energy and the dimensions and geometry of the ion guide electrodes. In certain implementations disclosed in the foregoing patent application, the magnitude or strength of this radial DC deflecting field is constant along the ion flight path, i.e., through the ion guide from ion entrance to ion exit.
A radial DC deflecting field that is constant along the ion flight path works well for evacuated or low-pressure ion guides. However, a constant DC deflecting field may not work well for ion guides in which ions lose a significant amount of kinetic energy as they travel through the ion guide, and/or for ion guides in which lower-mass ions are formed in the ion guide and require less deflecting forces than other ions of higher mass that also must be controlled in the same ion guide. Such conditions occur, for example, in ion guides utilized as collision cells and similar devices. The theory, design and operation of collision cells and similar devices are well-known to persons skilled in the art and thus need not be detailed in the present disclosure. Typically, a collision cell is an ion guide that is filled with a neutral gas and may serve as a primary ion optical component of a tandem mass spectrometer and in particular a triple quadrupole mass spectrometer. A collision cell is mainly employed to perform the function of MS/MS or collision-induced dissociation (CID). A collision cell may be curved as discussed above for the general case of ion guides, and a curved collision cell presents similar challenges. In addition to those challenges, in collision cells the ions experience a significant number of collisions with the background gas pursuant to the intended performance of CID or ion fragmentation. Thus, the kinetic energy of these ions decreases continually along the flight path. Moreover, the product ions that are formed as a result of ion-gas molecule collisions have a lower mass and a lower energy than their corresponding precursor ions, such that the product ions require less or no radial deflecting field in order to be successfully contained in the collision cell. It can be seen, then, that a constant DC deflecting field may not provide optimized transmission for all of the various ion masses typically processed in collision cells and like instruments.
Accordingly, there continues to be a need for improved curved ion guides in which ion transmission conditions are optimized, including ion guides such as collision cells in which ions experience appreciable losses of kinetic energy and ions of significantly different masses require deflection.